Maybe instead of simple feedback it would be worth to just task the developer with improving something that they will see as problematic (kind of 20% time but for improvement).
Whatever you say, it can only make it worse.
Now, let me start by saying I don't intend for this to be a suggestion for everyone in every situation, I just want to suggest that sometimes it can make things better.
I left my job three months ago, but I had actually tried to quit about 6 months earlier, instead I ended up switching positions. None of the problems I was having really ended up getting resolved after the my first attempt to quit, which is why I ended up actually leaving after giving it 6 months. But from what I've heard from friends still there, since I left they've actually been trying to fix things.
For the last month or so, my boss knew I was again debating leaving and we ended up having at least one conversation a week about things that were wrong. For me personally, it was great, I didn't have that stupid voice in the back of my head telling me to downplay everything. I think he appreciated how candid I was too.
Now don't get me wrong, I'm not saying everyone should always do this. I worked for a company with ~150 people and I was the 12th hire, with my boss coming in shortly after me. However, saying you should never do it isn't true either.
Much happier now, it's worked out well for both of us.
But if people have problem with feedback why are they asking for it?